This invention concerns an improved rod locking device for locking reciprocating piston type fluid actuators in a preferred position. Actuators of this type are particularly suited for positioning the rams of ram-type oilfield blowout preventers. It is advantageous and desirable to mechanically lock the rods of these actuators thus ensuring that the rams of these blowout preventers will be maintained in their preferred position should hydraulic pressure be lost.
Prior rod locking devices of oilfield ram-type blowout preventers appear to reside in four types of devices. The first of these is typified by U.S. Pat. No. 3,353,455 which discloses the use of slip-like structures to grasp and lock the piston rod in place. Hydraulic means are provided for releasing these slip-like structures at the desired time. A similar structure is found in U.S. Pat. No. 3,470,793 wherein the slip-like structure is retained in a groove on the piston rod and the slips wedge a conically shaped friction liner outwardly against the piston housing to lock the piston rod. Another example of this type of device is disclosed in U.S. Pat. No. 3,941,141 wherein the slip-like structures are provided with friction engaging surfaces in the form of sharp teeth used to grasp a corresponding specially serrated surface on the piston rod of the ram-type blowout preventer.
The second type of rod locking device is disclosed in U.S. Pat. No. 3,242,826 in which a plurality of arcuate segments engage a mating groove in the wall of the piston housing to lock the piston rod. This structure is limited in that the locking of the rod can occur only at a limited number of discrete point in the rod's travel.
A third type of rod locking devices utilizes a pair of frictionally engaging discs which selectively lock and unlock an overhauling nut by a combination of mechanical and hydraulic means. This overhauling nut threadingly engages the piston rod of the ram-type blowout preventer and allows the rod to be locked in a desired position. Typical examples of these devices can be found in U.S. Pat. Nos. 4,052,995 and 4,076,208 and 4,290,577.
U.S. Pat. Nos. 3,208,357 and 4,601,232 disclose the fourth type of structure which locks the rod in position by wedging means acting on the end of the tail rod with a hydraulic release means. These wedging type of lock devices are the simplest type of structure except that they require some additional locking structure or the wedge must be substantially elongated with a very shallow angle to provide a locking taper.